Q - "From a friend who likes philosophy - Think of an apple on the table. You and your friend see the apple. You will never be able to describe it exactly the same way because you both experience the apple differently. We still understand that there is only one apple; how is a God any different? Why can we differ on our perception of an apple yet still both be talking about the same apple, and not both be talking about the same God (Christian, Muslim, Jewish, etc.)? First, because I can measure the apple. Second, the physical apple is the same, but the conception and perception is different. In the case of a God, there is no physical substance. Your conception of God is limited to your own subjective experience.....which leads to the question: Can we objectively demonstrate the existence of God?"
A - Thanks for the questions.
First - can we objectively prove God's existence? To say yes, would mean that God is scientifically measurable as an apple is. So, in that sense, certainly not. But, there are arguments for God's existence that are rational and objective in a philosophical sense. Not, that we prove God's existence beyond a shadow of a doubt or that we prove it in the way science proves the existence of the apple, but rather we can show that a belief in God's existence is rational and true. To scientifically prove God would make God into less than God.
Second - is our concept of God subjective? In a certain sense it is and in a certain sense it is not. Each of us experiences life as an individual subject. But, this subjectivism does not mean that there can therefore be no objective knowledge. If this is the case, then there is no universal truth. Yet, for Christians, we believe that God objectively reveals Himself to us through His Son Jesus Christ - who is The Truth.
Truth is independent of our conceptualization of it or our experience/knowledge of it. Truth is.
It seems your friend has been reading quite a bit of Kant or Hume. The problem with there agnosticism is that they deny truth or our ability to know it. But, for a Christian, we would argue for the simple law of non-contradiction. Not all religions can be true if they contradict one another. How can Christianity / Islam / Judaism all be the same when we have exclusive claims about the nature of God. Jesus is or isn't divine. The truth about Him cannot be both. In examining the evidence surrounding Christ, I believe Him to be the Truth itself - a divine truth revealed to us objectively.